In the 3rd generation (3G) mobile communication system and the 4th generation (4G) mobile communication system, high rate data transmission is available. In addition, higher reliability requirements are imposed on these systems, especially under harsh natural conditions. Thus, the data retransmission technology becomes one of critical technologies meeting the high reliability requirement.
The data retransmission technology is a time diversity technology in essence, and can improve the performance of radio communication systems. Besides the time diversity technology, multiple diversity technologies are available. The principles of these diversity technologies are the same as the principle of the time diversity technology. That is, after a group of data is transmitted on one diversity, the group of data is retransmitted one time or multiple times on another diversity. Common diversity technologies include the time diversity technology, cell diversity technology, polarization diversity technology, frequency diversity technology, space diversity technology, code diversity technology, and relay diversity technology. To fully use the time domain resource, frequency domain resource, or code domain resource of a channel, a hierarchical modulation technology is introduced in radio communication technologies. The hierarchical modulation realizes better coverage with an enhanced technology while guaranteeing the most basic receiving quality, thus providing a better receiving effect. In hierarchical modulation mode, a receiving end with a better signal to noise ratio (SNR) receives data streams of two layers concurrently, while a receiving end with a poor SNR receives only the data of the basic layer.
The following describes the hierarchical modulation technology by taking a multimedia broadcast multicast service (MBMS) as an example. To meet SNR requirements of different receiving ends without occupying additional spectrum resources, a same data stream is divided into multilayer data for modulation, and then the modulated data is transmitted. For better description, the following takes a two-layer data stream as an example. The MBMS server transmits information b0b1b2b3 to the receiving end through a base station (BS). Herein, b0b1 indicates the basic layer data in the hierarchical modulation and is mapped to a quaternary phase shift keying (QPSK) symbol x. b2b3 indicates the enhanced layer data and is mapped to a QPSK symbol y. x and Y are multiplied by the corresponding modulation factors (assumed to be α and β, where
            α      =                                                  2              ⁢                              2                                                    10                                ⁢                                          ⁢          and          ⁢                                          ⁢          β                =                              2                                10                                )    ,and are superposed into a 16-state quadrature amplitude modulation (QAM) symbol z (z=αx+βy), and then the 16-state QAM symbol z is sent. When the BS actively retransmits the MBMS, the BS still modulates the basic layer data and the enhanced layer data into z=αx+βy in hierarchical mode.
The receiving end with a higher SNR receives both the basic layer data and the enhanced layer data. The receiving end with a lower SNR receives only the basic layer data. If the receiving end with a lower SNR also receives the enhanced layer data, the receiving end may also enjoy better quality of service (QoS). After receiving the transmitted data, the receiving end demodulates the basic layer data and the enhanced layer data, and then processes the data. Demodulating the received data twice may obtain a bit class logarithm likelihood ratio (LLR) value that is transmitted twice, which includes the basic layer data and the enhanced layer data. The signal of the transmitting end may be restored through combination and decoding of the LLR values, where the combination may adopt maximal ratio combining (MRC) or selective combining (SC) algorithms.